Power lines (also known as "conductors") are supported by power line poles, which may be wooden, metal or other typically used materials. The power lines, such as electrical transmission or distribution lines, are mounted to primary insulators. Primary insulators are typically made of a ceramic material or a synthetic polymer material and have various shapes and designs depending on the required voltage rating. The interior of the primary insulator is typically threaded in order to mate with a threaded element (also known as an "insulator thread") in accordance with the dimensions specified by ANSI C 135.17 (1988), either for a one inch or a one and three-eighths inch thread.
The threaded element, with which the primary insulator mates, is typically formed on a pin, which is directly or indirectly mounted to the power line pole. As used herein, the term "pin" includes any conventionally used rod-like element adapted for insertion into the interior of a threaded element for a primary insulator. Known pins include brackets, spacers, attachments and the like. Pins are usually metal or fiberglass or fiberglass with metal ends and can be mounted at the top of a power line pole (i.e. pole-top pin) or on the side of a power line pole (i.e. side pole pin).
There are a variety of ways to mount a pin to a power line pole. For example, FIG. 4 shows a metal pin 41 having a square neck 46 and a flat circular head 48 for an increased bearing surface. Flat head 48 rests against the top of a wooden cross arm while square washer 52 bears against the bottom of the wooden cross arm by tightening square nut 54 along straight threading 50. Alternatively, FIG. 1 shows a cast metal pin 18 which can be mounted to a power line pole in a known manner. For example, a fiberglass rod (not shown) is firmly mounted to the power line pole at one end by using a base structure, and the other end of the rod fits within bore 16, which is defined by perimeter 14.
Regardless of whether a metal pin or a fiberglass pin is used, a threaded element having dimensions in accordance with ANSI C 135.17 (1988) must be affixed to the top of the pin for engaging and supporting a primary insulator. For example, FIG. 4 shows threaded element 42, and FIG. 1 shows threaded element 20. This threaded element must be of a material which is sufficiently pliable so that the primary insulator, which is typically a brittle material such as ceramic, does not fracture upon engagement with the threaded element. Additionally, to reduce manufacturing costs and to improve the ability of the threaded element to be manufactured, the material of the threaded element preferably has a low melting point.
After mounting the primary insulator on the threaded element, the assembled unit must be resistant to rotational and tensile forces. Such forces can be caused by movement, or galloping, of the power line as a result of wind, or sudden dropping of ice or snow from the power line, or other forces. Excessive rotational forces could inadvertently be applied to the unit during installation of the primary insulator on the threaded element. Finally, the threaded element is preferably self-lubricating or easily conforming to the contour of the internal insulator thread to facilitate installation of the primary insulator on the threaded element.
To meet these needs, lead has been used in the industry as the material for the threaded element. Lead has a low melting point, is pliable and is self-lubricating. However, lead has been listed as a hazardous material by the Environmental Protection Agency and other authorities. Therefore, there is reason to avoid the use of lead as a material for the threaded element.
As an alternative to lead, composite plastic materials have been used as the material for the threaded element. For example, FIG. 4 shows a known apparatus which includes plastic threaded element 42. In this known apparatus, coarse threading 44 is formed on the top of metal pin 41. Threaded element 42 is directly molded in place over coarse threading 44 of metal pin 41. Thus, after curing, threaded element 42 is held in place by the interlocking of coarse threading 44 and the corresponding threading formed at the inner surface of threaded element 42 during the molding and curing process.
Efforts have been made to develop threaded elements capable of being mounted on metal or fiberglass pins which do not have coarse threading formed thereon. One such effort involves forming a threaded element with an inner diameter which increases along its length from the top to the bottom of the threaded element. Such a threaded element would have a generally constant thickness along its length because the above-mentioned ANSI specification requires that the outer diameter of the threaded element also increase along its length from top to bottom. An adhesive resinous material is placed between the inner diameter of the threaded element and the outer surface of the pin. In such a system, the point of least resistance to an upward tensile force on the primary insulator (and thus on the threaded element) is the relatively weak bond between the threaded element and the epoxy.
There are various known methods for manufacturing a metal pin. These methods include metal casting, metal forging, and metal forming. Typically, steel is used as the metal. For example, a steel pin could be formed from carbon steel in accordance with ASTM-A-36 and hot-dipped galvanized steel according to ASTM-A-153 (class "B"). Metal forging results in a pin manufactured to close tolerances, while metal forming and casting result in a large variance of tolerances. Typically, further machining of a formed or cast steel pin is required prior to mounting a threaded element thereon. To date, there are no known successful methods for mounting a threaded element on a formed pin without any additional processing, such as machining or turning.